1. Field of the Invention
The present invention relates to a contact detecting apparatus for detecting the contact of a tool with a workpiece when they are moved relatively to one another.
2. Description of the Prior Art
As shown in FIG. 1, conventional contact detecting apparatus is provided with a coil 12 mounted on the end of a spindle head 11 in which a spindle 13 is rotatably received. This coil 12 is connected to an AC power source 14 to be energized, so that the coil 12 generates loops of magnetic flux surrounding the spindle 13 which induces current in a direction axially of the spindle 13. When a tool 1 does not contact a workpiece 2, a looped secondary circuit 15 connecting the spindle 13 and a machine body 10 is not completed, whereby there is no induced current. On the other hand, when the tool 1 contacts the workpiece 2, the looped secondary circuit 15 is completed to produce induced currrent. In this way, the secondary circuit 15 is completed depending upon the contact between the tool 1 and the workpiece 2, and an exciting current in the coil 12 on the primary side is changed depending upon the change in electric current in the secondary circuit 15. Accordingly, the contact between the tool 1 and the workpiece 2 can be detected by connecting the coil 12 and a resistance R1 in series and detecting the change in voltage appearing at terminals a and b across the resistance R1 by a voltage detector 16. FIG. 2 shows an equivalent circuit of the arrangement shown in FIG. 1. In FIG. 2, the secondary circuit 15 constitutes a winding of one turn; the tool 1 and the workpiece 2 constitutes a switch SW opened and closed by the contact.
In such an arrangement, when the spindle 13 was not rotated, the voltage change appeared on the terminals a and b depending upon the contact between the tool 1 and the workpiece 2, whereby the contact could be detected. However, when the spindle was rotated, substantially no voltage change appeared on the terminals a and b, so that the detection of the contact was difficult or virtually impossible.
It was found by inspection that an electric resistance R2 in the secondary circuit 15 was changed from several hundred m.OMEGA. to several hundred .OMEGA. depending upon rotation of the spindle, lubrication of spindle bearings, preload of the spindle bearings, and so on, and that this resistance R2 was changed mainly depending upon the electric resistance change of the bearings supporting the spindle 13, although also depending upon the contact resistance between the tool 1 and the workpiece 2 to some extent. When this resistance R2 was changed, the ratio of the voltage V1ON appearing on the terminals a and b when the switch SW is closed to the voltage V1OFF when the switch is opened, changed as shown in FIG. 3. In this case, the frequency f of the power source was 20 KHz, the inductance L of the coil 12 was 159 mH, the turn of the coil was 159 T, the winding resistance r1 was 0.35.OMEGA. and the detecting resistance R1 was 100.OMEGA.. According to the characteristics shown in FIG. 3, when the resistance R2 in the secondary circuit 15 is quite small, the voltage change ratio becomes greater than 2. However, when the resistance R2 is increased, the voltage change ratio is abruptly lowered to about 1, resulting in difficulty in detection. Taking change in power source voltage, drift of an amplifier and the like and induction noise into consideration, detection of the contact is considered to be difficult, unless the voltage change ratio is above 1.1. Accordingly, it is realized that the resistance R2 in the secondary circuit 15 has to be set below about 1.OMEGA..
Roller bearings or ball bearings are usually used as spindle bearings. It is presumed that when the spindle is not rotated, rolling members such as rollers or balls make metallic contact with rolling surfaces of inner and outer races of the bearings, so that the electric resistance is small. It is also presumed that when the spindle is rotated, an oil film is formed between the rolling members and the rolling surfaces to prevent metallic contact, so that the electric resistance becomes large.